Electret microphone

ABSTRACT

The electret member of an electret microphone is a substantially cylindrical body of barium titanate composition having a first flat end face opposite the microphone diaphragm and a second end face covered with a fixedly attached electrode which extends into a blind axial bore centered in the second end face. The electret member is attached to a supporting plate by a bolt soldered to the electrode in the bore and fastened to the support plate by a nut. The bolt provides the conductive lead for the electret.

United States Patent Inventors Appl. No.

Filed Patented Assignee Priorities Vlastimil Koukal; I

Otokar Krakora, both of l-lradec Kralove, Czechoslovakia Aug. 4, 1969 Nov. 30, 1971 Tesla, narodni podnik Prague, Czechoslovakia Oct. 24, 1964 Czechoslovakia Oct. 26, 1964, Czechoslovakia, No.

595 l/64 Continuation-impart of application Ser. No. 495,452, Oct. 13, 1965, now Patent No. 3,474,197, dated Oct. 21, 1969. This application Aug. 4, 1969, Ser. No. 847,379

ELECTRET MICROPHONE 2 Claims, 3 Drawing Figs.

u.s. c1 179/111 E, 307/88 ET Int. Cl ..H04rl9/04, H04r 23/00 [50] Field of Search l79/l ll E; 307/88 ET; 252/632 [56] References Cited UNITED STATES PATENTS 3,354,373 l l/l967 Fatovic 320/1 OTHER REFERENCES Experimentelle Technik der Physik, Vol. 10, No. 3, 1962 Primary Examiner-Kathleen H. Claffy Assistant Examiner-Thomas L. Kundert An0rneyRichard Low ABSTRACT: The electret member of an electret microphone is a substantially cylindrical body of barium titanate composition having a first flat end face opposite the microphone diaphragm and a second end face covered with a fixedly attached electrode which extends into a blind axial bore centered in the second end face. The electret member is attached to a supporting plate by a bolt soldered to the electrode in the bore and fastened to the support plate by a nut. The bolt provides the conductive lead for the electret.

PATENTEUNUNOIB?! 3,624,312

INVENTORS VLnsT/mH h ML g'wKnR K Koafl mme ATTORNEY ELECTRET MICROPHONE REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of our copending application Ser. No. 495,452, filed Oct. 13, 1965, and now U.S. Pat. No. 3,474,197 dated Oct. 21, 1969.

BACKGROUND OF THE INVENTION This invention relates to electret microphones.

The electret member of a conventional electret microphone is usually a circular or elliptic disc which may be provided with holes for modifying the electroacoustic qualities of the microphone. All parts of the known microphone are fixedly mol percent lead titanate, 0.5 to [5 mol percent calcium titanate, 0.1 to 2 percent magnesium oxide, 0.5 to 8 mol percent lead stannate, 0.2 to 10 mol percent calcium stannate,

and 0.4 to 5 mol percent bismuth stannate.

In another group of electret compositions of the invention there are 92 to 98 mol percent barium titanate, 0.5 to 4 mol percent lead stannate, l to 7 mol percent calcium stannate, and 0.5 to 2.5 mol percent bismuth stannate.

Yet another group of compositions particularly suitable for the electret members of the invention include 77 to 90 mol percent barium titanate, 3 to 6 mol percent lead titanate, 6 to mol percent calcium titanate, and 0.5 to 2 mol percent magnesium oxide.

Other features and many of the attendant advantages of this mounted in a housing by means of circular rings. The electret 15 member must be insulated very carefully from the other parts lIWeht-wn lll readily come apparent from the following of the microphone because it provides a very high output im- F descnpu? a Preferred embodmfem when pedance whose real component may reach a value of up to 10 Sdered connecuon the appended ohms. Even the smallest traces of impurities or moisture or a leak in the insulating rings mayimpair the operation of the BRIEF DESCRIPTION OF THE DRAWING microphone. The setting of the airgap between the diaphragm I i n the drawing. r g g g grs member ls cmlca] for good operanon of the FIG. 1 shows an electret microphone of the invention .in I The known electret microphones are not fully satisfactory. frlagmemary i zf and partly m Section on the axls of the The supporting members for the electret must be made of e f g g r l t t b f th mic hone of ceramics which must be fired, and which shrink during firing FIG i 5; g f z i g g f z a so that they must be finished to the required close tolerances w ofthe lefiet member by laborious grinding after firing. Microscopic pores between IS a P p th l l t t the electret member and the associated electrode interfere n y as 0 o emflse qfnvenuona e ec re with the electric as well as the mechanical contact between mlcroph? 18 Show P the fhawmg as Feeded for an these elements. The mechanical stability of the known ardrsmndmg the f The mcrophom a rangements is not adequate, and it is very difficult to set and fjlaphragm 1 as co'fvemlonal' An electIet mem?-er maintain an airgap of optimum dimensions between the mema generally cylmdncal of ceramic bafmm t f brane and the electret member composition, is mounted in the microphone hous ng, shown in Although electret microphones have been known for many Phantom View in Such a l l? the axiaux terminal l years, and although they offer recognized advantages over face of the electret member F d y opposlte t other microphones, the production and use of microphones bl'ahe The bottom fatie 0f the l t member 2 15 o e ed e ui ed with inor ani l tret h be very li it d by a fixedly attached metal electrode 3 held in abutting enbecause of the aforementioned difficulties. The object of the g g m with a pp i g Plate 4 y a l 5 and a hut invention is the provision of an electret microphone which is 40 thrcadedly m nte n th ltfree from the shortcomings of known electret microphones The top face of the electret member 2 is formed with eight enumerated above. elongated groves 7 which extend radially inward from the periphery of the top face and are equiangularly distributed SUMMARY OF THE INVENTION about the circumference of the electret member. A blind axial wi i object and others in View, as i h i f -bore8is centered in the bottom face of the electret member 2. become apparent, this invention provides an improvement in a Its Walls ate covered y the metal Q the electrode and the mi h i d i h a di h an l q b bolt 5 is received in the bore 8 and soldered to the electrode spacedly opposite the diaphragm, a housing enclosing the metalih elecmcany conductive engagement: diaphragm and the electret member, and conductors for con- The grooves 7 modify the electfoacoustlcal P p of necting the diaphragm and the electret member to an electric the m p n Conductors, not Q Connect the circuit. According to this invention, the electret member is a diaphragm l and the bolt 5 to an amplifier circuit external to substantially cylindrical body of ceramic material which has a the microphone proper, and the bolt 5 thus constitutes an elefirst axially tenninal face opposite the diaphragm and a second ment of the conductive lead arrangement of the microphone. axially terminal face directed away from the diaphragm. The The inorganic electret member 2 may consist of a convenfirst-mentioned face of the electret member is formed with a tional barium titanate composition or other permanently plurality of elongated grooves. An electrode substantially polarized dielectric material, but best results have been covers the second terminal face of the electret member and is achieved with compositions which generally fall within the fixedly fastened to the face. The electrode is held in abutting mol percentage range of 70-98% BaTiO 06% PbTiO engagement with a supporting member in the microphone l5% caTio ()-2% M O, O-8% Pbs o 040% Cas o housing y fastening means which also fasten the electret and 0-5% Bi sno,),. The compositions listed in the following member to the supporting member. table have been found to be particularly effective:

" TABLE M01 percent Example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Ba'IiO; 77-90 94 95 92 70 70 32 PbTi0;, 3-6 6 0.3 4.5 2 CaTiOg- 6-15 15 15 0.5 7 PbSnOa. 2.5 0.5 0.5 0.5 s 5 CaSnO; 2.5 3.5 7 8 10 0.2 BI1(SIIO3)3 1. 0 1. 5 0. 5 0. 4 4. 1 5 2. 8 Mg() 0.5-2 0.1 0.1 3 1 This invention also resides in electret compositions of which When assembling the electret microphone of the invention,

one consists of 70 to 98 mol percent barium titanate, 0.3 to 6 it is of advantage to place the unpolarized electret member in the microphone housing so as to set the optimal airgap between the diaphragm and the electret member, whereupon the electret member is polarized in the usual manner and the microphone housing is closed. If the electret member were polarized before being installed in the housing of the microphone, it could be contaminated and short-circuited during mounting and especially during the setting of the airgap between the diaphragm and the electret member. The sensitivity of the microphone could be reduced significantly.

The electret microphone according to the invention avoids the inadequacies of the known devices as described above, its structure and assembly are simple, no holes need be made in the electret member, and the connection between the electret member and the electrode cannot be impaired. The electret microphone of the invention is particularly resistant to effects of the atmosphere and to mechanical damage.

We claim:

I. In a microphone having a diaphragm, an electret member spacedly opposite said diaphragm, a housing enclosing said diaphragm and said electret member, and conductive means for connecting said diaphragm andsaid electret member to an electric circuit, the improvement which comprises:

a. said electret member being a substantially cylindrical body of ceramic material and having a first axially terminal face spacedly opposite said diaphragm, and a second axially terminal face directed away from said diaphragm,

b. said electret member being formed with a plurality of elongated grooves in said first face,

c. said electret member essentially consisting of 70 to 98 mol percent BaTiO 0.3 to 6 mol percent PbTiO 0.5 to mol percent CaTiO 0.l to 2 mol percent MgO, 0.5 to

8 mol percent PbSn0 0.2 to 10 mol percent CaSn0,, and 0.4 to 5 mol percent Bi,(SnO,)3,

d. a supporting member in said housing,

e. an electrode substantially covering said second face and fixedly fastened to said second face, and

f. fastening means fastening said electret member to said supporting member in abutting engagement of said electrode with said supporting member.

2. In a microphone having a diaphragm, an electret member spacedly opposite said diaphragm, a housing enclosing said diaphragm and said electret member, and conductive means for connecting said diaphragm and said electret member to an electric circuit, the improvement which comprises:

a. said electret member being a substantially cylindrical body of ceramic material and having a first axially terminal face spacedly opposite said diaphragm, and a second axially terminal face directed away from said diaphragm,

b. said electret member being formed with a plurality of elongated grooves in said first face,

c. said electret member essentially consisting of 92 to 98 mol percent BaTiO 0.5 to 4 mol percent PbSnO l to 7 mol percent CaSn0 and 0.5 to 2.5 mol percent Bi (SnO b83.

d. a supporting member in said housing,

e. an electrode substantially covering said second face and fixedly fastened to said second face, and

f. fastening means fastening said electret member to said supporting member in abutting engagement of said electrode with said supporting member.

' t a a 

1. In a microphone having a diaphragm, an electret member spacedly opposite said diaphragm, a housing enclosing said diaphragm and said electret member, and conductive means for connecting said diaphragm and said electret member to an electric circuit, the improvement which comprises: a. said electret member being a substantially cylindrical body of ceramic material and having a first axially terminal face spacedly opposite said diaphragm, and a second axially terminal face directed away from said diaphragm, b. said electret member being formed with a plurality of elongated grooves in said first face, c. said electret member essentially consisting of 70 to 98 mol percent BaTiO3, 0.3 to 6 mol percent PbTiO3, 0.5 to 15 mol percent CaTiO3, 0.1 to 2 mol percent MgO, 0.5 to 8 mol percent PbSnO3, 0.2 to 10 mol percent CaSnO3, and 0.4 to 5 mol percent Bi2(SnO3)3, d. a supporting member in said housing, e. an electrode substantially covering said second face and fixedly fastened to said second face, and f. fastening means fastening said electret member to said supporting member in abutting engagement of said electrode with said supporting member.
 2. In a microphone having a diaphragm, an electret member spacedly opposite said diaphragm, a housing enclosing said diaphragm and said electret member, and conductive means for connecting said diaphragm and said electret member to an electric circuit, the improvement which comprises: a. said electret member being a substantially cylindrical body of ceramic material and having a first axially terminal face spacedly opposite said diaphragm, and a second axially terminal face directed away from said diaphragm, b. said electret member being formed with a plurality of elongated grooves in said first face, c. said electret member essentially consisting of 92 to 98 mol percent BaTiO3, 0.5 to 4 mol percent PbSnO3, 1 to 7 mol percent CaSnO3, and 0.5 to 2.5 mol percent Bi2(SnO3)3. d. a supporting member in said housing, e. an electrode substantially covering said second face and fixedly fastened to said second face, and f. fastening means fastening said electret member to said supporting member in abutting engagement of said electrode with said supporting member. 